Add OS X/Swift and Python implementations of the Eight Queens problem

EightQueens-python/EightQueens.py

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+# Eight Queens
+# a solution to the Eight Queens problem written in Python
+# by Donald Burr <dburr@borg-cube.com>
+#
+# more on the Eight Queens problem:
+# http://en.wikipedia.org/wiki/Eight_queens_puzzle
+
+import cProfile
+
+# a simple class to represent a Queen
+class Queen:
+    def __init__(self, row):
+        self.row = row
+        self.column = 0
+
+# Array to hold our 8 queens
+# we're starting off by placing each queen on a separate row
+queens = []
+for row in range(8):
+    queens.append(Queen(row))
+
+# some variables to keep some interesting statistics
+solutions_found = 0
+positions_checked = 0
+
+# test the queen at a given row to see if it is safe at a given column
+def is_safe(current_row, current_column):
+    global positions_checked
+    global queens
+    
+    positions_checked = positions_checked + 1
+    
+    for previous_row in range(current_row):
+        # check vertically: are we trying to place in the same column as a previously placed queen?
+        if queens[previous_row].column == current_column:
+            # bzzt - not safe!
+            return False
+        
+        # now check diagonally
+        difference_in_rows = current_row - previous_row
+        difference_in_columns = current_column - queens[previous_row].column
+        if difference_in_rows == difference_in_columns or difference_in_rows == -difference_in_columns:
+            # bzzt, not safe!
+            return False
+            
+    # we've compared against all queens, so we must be good
+    # place the queen and we're outta here!
+    queens[current_row].column = current_column
+    return True
+    
+# recursively try moving a queen across all columns
+def move_queen_across_row(row):
+    global solutions_found
+    
+    for column in range(8):
+        # move queen column by column, checking if it's safe
+        if is_safe(row, column):
+            # if we just placed the last queen, we've got a solution
+            if row == 7:
+                solutions_found = solutions_found + 1
+                print_answer()
+            else:
+                # recursive call - now move the queen to the NEXT row
+                move_queen_across_row(row+1)
+
+# print out a solution once we've found it                
+def print_answer():
+    global solutions_found
+    global queens
+    
+    print "SOLUTION #%d\n" % solutions_found
+    # rows are in reverse order
+    for current_row in reversed(range(8)):
+        # print row number (using chess numbering)
+        print "%d" % (current_row+1),
+        for current_column in range(8):
+            if queens[current_row].column == current_column:
+                print " Q ",
+            else:
+                print " . ",
+        
+        print
+    
+    print "   A   B   C   D   E   F   G   H  \n\n"
+
+##### end of functions
+##### main code (such as it is) below here    
+##### putting it in a function so we can profile just for the halibut
+
+def main():
+    # kick things off by trying to move the first queen
+    move_queen_across_row(0)
+
+    # all solutions have been found
+    print "POSITIONS CHECKED: %d\n\n" % positions_checked
+    
+# the REAL start of the code
+# just profile it
+cProfile.run('main()')